1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit having a closed glass bulb in which a pair of electrodes are disposed, and a pinch seal on both sides of the closed glass bulb. In particular, the present invention relates to a mercury-free arc tube for a low-voltage discharge lamp unit having a closed glass bulb filled with a primary light-emitting metal halide, an optional buffer metal halide, and a starting rare gas.
2. Background of the Related Art
FIG. 3 illustrates a related art discharge bulb used in a discharge lamp unit as a light source of vehicular lamps. The related art discharge bulb includes an arc tube 2 containing a closed glass bulb 2a (or a light emitting portion) formed integrally with a resin insulating plug body 1. The rear end of the arc tube 2 is supported by a metal support 8 that is fixed to the insulating plug body 1, and the front end of the arc tube 2 is supported by a metal lead support 9 that extends from the insulating plug body 1 and serves as a current path.
The arc tube 2 is pinch-sealed at both ends by pinch seals 2b. The closed glass bulb 2a has opposing electrodes 3 extending into the ends of the closed glass bulb of the arc tube 2, which is filled with a metal halide as a primary light-emitting substance, mercury as a buffer gas, and a rare gas as a starting gas.
Mercury in the arc tube is the buffer for maintaining a prescribed tube voltage and buffering (i.e., reducing electron collision against the electrodes) to reduce electrode damage. Mercury is also an auxiliary light-emitting substance for generating white light.
In the related art discharge bulb, light is emitted from an arc discharge created between the electrodes 3. Since arc tubes produce more light and have a longer service life than incandescent lamps, they are being increasingly used as a light source of headlights or fog lights.
In the related art discharge bulb illustrated in FIG. 3, a lead wire 4 is located at an end of the pinch seal 2b. A pinch-sealed molybdenum foil 5 is connected between a lead wire 4 and a tungsten electrode 3. The arc tube 2 is integrally welded to a glass shroud 6 that provides ultraviolet shielding and creates a closed space that includes the foregoing elements. The glass shroud 6 blocks ultraviolet rays in a wavelength range harmful to human body from being emitted by the arc tube 2, and keeps the glass bulb 2a at a high temperature.
However, the foregoing related art discharge bulb has various problems and disadvantages. Because the related art closed glass bulb 2a contains mercury as a buffer gas, the related art mercury discharge bulb is harmful to the environment. To overcome the foregoing problem and meet the recent social demand for reduction of environmental pollutants, there has been a need to develop an arc tube containing no environmentally harmful mercury, namely a mercury-free arc tube.
Unfortunately, the foregoing related art bulb could not exclude mercury for at least the following reasons. For example, but not by way of limitation, excluding mercury from the closed glass bulb would reduce tube voltage and require an increased electrical current for maintaining the tube voltage. As a result, there would be an increased load on the electrodes, leading to a reduction in luminous efficiency. Elimination of mercury from the closed glass bulb 2a also prevents generation of light having a desired chromaticity.
To overcome at least the foregoing related art problems, applicants used a metal halide that acts as a buffer in place of mercury and realizes the same chromaticity as the foregoing related art mercury-containing arc tubes, to create a mercury-free arc tube exhibiting similar characteristics to those of the mercury-containing arc tubes, with no alterations to the shape and dimensions. This mercury-free arc tube includes a closed glass bulb filled with a primary light-emitting metal halide, together with the selected buffer metal halide and a starting rare gas. The pressure of the rare gas is about 8 to 20 atm, which is higher than in the related art mercury-containing arc tubes (3 to 6 atm). Applicants filed Japanese Patent Application No. 2001-286252, the contents of which is incorporated herein by reference.
A further modification of the foregoing mercury-free arc tube produced characteristics similar to those of related art arc tubes with no buffer metal halide sealed into the closed glass bulb. The total amount and proportion of a prescribed primary light-emitting metal halide and the pressure of the starting rare gas (about 8 to 20 atm) sealed into the closed glass bulb was maintained. Based on this modification, applicants filed Japanese patent Application No. 2002-243489 (not prior art), the contents of which is incorporated herein by reference, claiming priority to the above-noted Japanese Patent Application No. 2001-286252.
The mercury-free arc tube of JPA 2002-243489 has similar characteristics to related art mercury-containing arc tubes, and a power consumption of about 35 W, which is the same as the related art mercury-containing arc tubes.
However, users demand a reduction in the energy required to operate discharge bulbs (arc tubes). Also, the number of electric parts and accessories mounted on recent automobiles has been increasing, which has increased the total power consumption. The increased power consumption and attendant increase in harness length and weight have made it difficult to reduce vehicle fuel consumption. Although the power consumption of discharge bulbs using an arc tube as a light source (about 35 W) is lower than that of halogen lamps (about 60 W), it is still higher than most automobile electric parts and accessories. Therefore, there is a need to reduce power consumption of discharge bulbs.
To address the foregoing need to save power in a discharge bulb having the structure of JPA 2002-243489, applicants tested a reduction in electric current supplied to the arc tube from its rated value to decrease the ordinary power consumption during steady lighting from 35 W to 25 W. As a result, the tube voltage decreased from 42 V to 40 V, the tube current decreased from 0.830 A to 0.600 A, the luminous flux decreased from 3200 lm to 2000 lm, and the luminous efficiency decreased from 91 lm/W to 80 lm/W. The chromaticity also decreased.
Accordingly, applicants obtained the following conclusions with respect to applicants' related art. First, a reduction in power consumption in a steady lighting mode reduces luminous flux, which in turn reduces luminous efficiency. As a result, the brightness of the lighting area is reduced.
Second, because a slight reduction in tube voltage results in a significant reduction in tube current, the electrode temperature drops. Thus, the required re-ignition voltage increases, and causes lamp flickering.
Third, the bulb emits bluish light due to the chromaticity change from x: 0.380 and y: 0.390, to x: 0.365 and y: 0.375.
As a result, there exists a need in the related art to overcome at least the foregoing problems and disadvantages.